A podcast board hints at a Hidden Blade
The board behind Aggrend in episode 6 of the WoW Forever podcast carries a line in Darnassian. Players translate its first word as Hidden Blade and suspect a secret weapon. Blizzard has not explained the text, and one word is still unknown.
SHEN’SERRAR LEZAL MELOR’NA ADALA FAL
The translation
A post on the official forums breaks the text down. Shen means hidden, known from the introduction of the Skyborne. Serrar means blade, as in Quel’Serrar. The phrase “Shanna melor’ne adal fal” means “The truth is a guiding light”, which covers the end of the line.
Only Lezal has no known meaning. The forum post suggests a reading like “The Hidden Blade is the guiding light” or “The Hidden Blade is our guiding light”.
The link with Onyxia
Onyxia’s Lair opens as a raid for 40 players on 9 December. In Classic, Warriors and Paladins got Quel’Serrar through a quest chain that involves Onyxia and starts with a compendium. In the Forever data, that book is 
Nostro’s Compendium of Dragon Slaying, and the sword itself is not there yet.
Secret hunters on Discord are looking into it, but with the game not live yet, there is little to test. Whether the line points to a weapon at all, is not known.
Source: Wowhead